礫組成からみた山陰東部地域の前期中新世末〜中期中新世古地理  [in Japanese] Latest Early to Middle Miocene paleogeography of the eastern Sanjn district, Southwest Japan, viewed from a provenance study of conglomerate  [in Japanese]

北但層群豊岡累層下部辻礫岩層の起源を同礫岩層の礫岩解析によって明らかにし, また, 北但馬地域を中心とした前期中新世末〜中期中新世古地理について議論した。辻礫岩層の砕屑物は分布域北西方および南東方に位置した2列の基盤から由来し, おそらく内陸で扇状地〜ファンデルタを形成した。基盤は, 主として白亜系〜古第三系の酸性火山岩類と下部中新統の安山岩, 玄武岩から構成され, 北東-南西〜東北東-西南西方向に伸長してbasinの方向を規制していた。堆積場は, 内陸環境から豊岡累層上部が堆積した汽水〜浅海環境を経て, 短期間のうちに急速に深海環境へと変化し, 厚い細粒砕屑物からなる村岡累層によって埋積された。豊岡累層と村岡累層を合わせた堆積速度は数100m/Myrに達した。

In the eastern San'in district, the uppermost Lower to Middle Miocene sediments are widely exposed. This paper presents sedimentological studies of the lower part of the sediments (Toyooka Formation), characterized by thick sequences of conglomerate, and a paleogeographic reconstruction of the district. The composition and size analysis of conglomerates from the Tsuji Conglomerate Member of the Toyooka Formation reveal that they were derived from two rows of basement highs, trending NE-SW and ENE-WSW, existed to the northwest and southeast of the sedimentary basin. The highs were composed mainly of Cretaceous to Paleogene felsic volcanic rocks and Neogene andesite and basalt, and controlled the elongate morphology (N'E-SW to ENE-WSW). The coarse clastic sediments were deposited under fresh-water conditions. They are interpreted as alluvial fan or fan delta deposits. Subsequently, the eastern San'in district submerged rapidly from a non-marine toa deep marine environment and the sedimentary basins became filled by thick accumulations of fine-grained deposits (Muraoka Formation) in a short period. The sedimentation rate was several hundred meters per million years during deposition of the Toyooka and Muraoka Formations.